Wet capacitors are used in the design of circuits due to their volumetric efficiency, stable electrical parameters, high reliability and long service life. Such capacitors typically have a larger capacitance per unit volume than certain other types of capacitors, making them valuable in high-current, high power and low-frequency electrical circuits. One type of wet capacitor is a wet electrolytic capacitor including an anode, a cathode, and a fluid electrolyte. Wet electrolytic capacitors tend to offer a good combination of high capacitance with low leakage current. Wet electrolytic capacitors are basic to various types of electrical equipment from satellites, aerospace, airborne, military group support, oil exploration, power supplies, and the like.
Known wet electrolytic capacitors are generally characterized as having a generally cylindrical shape and axial leaded terminations suited for Through-Hole Mounting (THM). Generally, tantalum electrolytic capacitors are known to have a general cylindrical shape and axial lead terminations suited for THM. FIGS. 1A and 1B show cross-sectional diagrams of an illustrative capacitor 100 having such an axial THM design. The capacitor 100 includes a generally cylindrical can-shaped body 105 formed from tantalum (Ta). The capacitor 100 includes an electrolyte 110 disposed in electrical contact with an anode 120 and cathode 130. The illustrated capacitor 100 includes a polytetrafluoroethylene (PTFE) bushing 140 at one end, surrounded by a rubber O-ring 150 that is positioned in a groove 145 the bushing 140. The end of the capacitor 100 is crimped 152 to compress the tantalum body 105 into the groove 145. Thus, the known axial capacitor of FIGS. 1A and 1B utilize a double seal construction consisting of a crimped PTFE plug lined with a rubber gasket serving as a primary seal, and a laser welded cover serving as a secondary seal. As can be appreciated, in such known capacitors, the PTFE bushing is located inside the interior area of the capacitor body or “can,” and thus takes up space from a limited volume that could otherwise be used for placement of a capacitive element.
THM assembly technology was standard practice for capacitors until the late 1980s, when Surface-Mount Technology (SMT), resulting in Surface Mount Devices (SMDs), largely replaced THM for a variety of cost and efficiency reasons. For example, THM requires the drilling of holes in the printed circuit board (PCB), which is expensive and time consuming. Component assembly speed for SMT is generally faster than that of THM because THM requires soldering on both sides of the board, as opposed to surface-mounts, which typically require attention to only one side of the PCB. THM assembly generally uses wave, selective, or hand-soldering techniques, which are much less reliable and repeatable than reflow ovens used for surface mounting. Furthermore, SMT components are generally smaller than its THM counterparts because they have either smaller leads or no leads at all.
One way to improve volumetric efficiency is to use a high performing material, for example, tantalum (Ta), Niobium (Nb), or Niobium Oxide (NbO), for the anode material. Certain solid core or pellet surface mount capacitors of this general type are known in the art. Examples can be seen at U.S. Pat. Nos. 6,380,577, 6,238,444, and 7,161,797, which are incorporated by reference herein. In those patents, examples show a solid interior core (sometimes called an anode body, slug or pellet) is primarily Ta. The tantalum anode body is usually sintered. A wire is commonly formed in the anode body in one of two ways: (a) “embedded” meaning the wire (which also can be tantalum) is covered with tantalum powder during a pressing process; or (b) “welded” meaning after the pellet is pressed and sintered, the wire is welded to the Ta slug. The other end extends outside the slug. The capacitor dielectric material is made by anodic oxidation of the anode material to form an oxide layer over the surface of the anode body (e.g., Ta to Ta2O5). If the anode body is Nb the oxidation is Nb to Nb2O5; if NbO, the oxidation is NbO to Nb2O5. A capacitor cathode is commonly formed by coating the dielectric layer with a solid electrolyte layer (e.g., of MnO2) and a conductive polymer, and later covered with graphite and silver for better conductivity and improved mechanical strength. Anode and cathode terminations can be connected to the free end of the Ta wire and the outer electrolyte surface coating of the Ta pellet, respectively, and all these components can then be encapsulated within a case (e.g., by molding plastic around the components), leaving only outer surface(s) of the anode and cathode terminations exposed on the exterior of the case for, e.g., surface mounting.
As can be appreciated, such known capacitors do not utilize a tantalum case or “can,” or a “wet” (fluid) electrolyte. Thus, they do not address the issue of volumetric efficiency when introducing a fluid electrolyte into a pre-formed tantalum case or can. They also do not address how to effectively seal such a case when the fluid electrolyte has been introduced.
There remains a need, then, for an improved wet electrolytic capacitor having a tantalum case, and in particular, for an improved wet electrolytic capacitor suitable for surface mounting and having improved volumetric efficiency. Further, there is a need for a capacitor having an improved construction for introducing an electrolyte into the interior of the capacitor body, without taking up valuable space in or on the capacitor body.